1. Field of the Invention
The present invention relates to an integrated active electronic filter having extremely low sensitivity to variations of the components, particularly for production in MOS technology.
2. Prior Art
It is known to provide active integrated filters, formed by resistors, capacitors and operational amplifiers, connected in synthesized networks to emulate passive filters made of resistors, inductors and capacitors.
It is also known, for the purpose of improving the precision of the response characteristics of the filter, to provide such integrated filters by replacing the resistors with switched capacitors, which can be obtained in integrated circuits with much greater precision than resistors. In such switched-capacitor filters and precision of the fundamental parameters depends in practice only on the precision of the capacity ratios, and is therefore very high.
For the design of such filters it is known to calculate a conventional passive step-like filter composed of resistors, capacitors and inductors, such that its frequency response complies with a desired mask, and to then obtain from said filter, with an also known procedure, a flowchart in which the components are replaced with the corresponding transfer functions according to Laplace's transform. Finally, synthesis methods are known to provide, starting from said flowchart, an active filter, constituted by operational amplifiers, capacitors and switches (to implement switched capacitors).
In the calculation of the passive step-like filter, it is known to minimize the sensitivity of the response to variations of the components by terminating the filter, at the input and at the output, with two identical resistors (Electronics Letters, vol. 2, pp. 224-225, June 1966, "Inductorless Filters"). This property is also conserved in active filters obtained from the step-like filter with double termination.
As is known to the expert in the field, the selectivity characteristics of a filter improve as the order of the filter rises, that is to say as the number of poles of the filter increases. In an active filter of the type described above, the number of poles is equal to the number of operational amplifiers present in the filter.
In the design of active filters in integrated circuits, the most costly part of circuit from the point of view of silicon area occupation is constituted by the operational amplifiers, and therefore filters are designed with the lowest possible order with regard to the required selectivity. Though it is generally possible to reduce by one the order of the filter, selectivity being equal, by eliminating the double termination, this is generally not done to avoid increasing the sensitivity of the filter.
In most applications, particularly in telecommunications, the frequency response specifications are met with the use of fifth-order filters, and therefore most of the filters produced comprise five operational amplifiers. If the double termination is eliminated, the same requirements of selectivity may generally also be met by a fourth-order filter, but this is not done in practice since it is more important to keep the sensitivity low.